1. Field of the Invention
This invention relates to the hydraulic fracturing of subterranean formations. In one aspect, the invention relates to the use of a non-damaging polymeric thickener for aqueous fracturing fluids capable of use in deep, high temperature wells.
2. Description of the Prior Art
Hydraulic fracturing has been widely used as a means for improving the rates at which fluids can be injected into or withdrawn from subterranean formations surrounding oil wells, gas wells, water injection wells, and similar boreholes. The fracturing methods employed normally involve the injection of a viscous fracturing fluid having a low fluid loss value into the well at a rate and pressure sufficient to generate a fracture in the exposed formation, the introduction of fluid containing suspended propping agent particles into the resultant fracture, and the subsequent shutting in of the well until the formation is closed on the injected particles. This results in the formation of a vertical, high-conductivity channels through which fluids can thereafter be injected or produced. The conductivity obtained is a function of the fracture dimensions and the permeability of the bed of propping agent particles within the fracture.
In order to generate the fracture of sufficient length and width and to carry the propping agent particles into the fracture, it is necessary for the fluid to have relatively high viscosity, low friction loss, and non-damaging properties. The viscosity in aqueous liquids is provided by the addition of polymers, frequently called thickeners. Following the treatment of the well, it is desirable to return the aqueous liquids to its low viscosity state to enhance cleanup, thereby permitting the fracturing fluid and polymer to be removed from the formation and the propped fracture. The highly viscous liquid if left in the fracture would reduce formation permeability and impede the production of formation fluids through the propped fracture. Moreover, the residue of the polymer on the fracture face and in the pores of the propped fracture would significantly reduce fluid permeability therethrough.
The polymers used as aqueous thickeners must impart sufficient fluid viscosity at the temperature of the formation to generate the desired fracture and suspend the proppant. The polymers are frequently crosslinked to achieve the necessary viscosity. One problem associated with some crosslinked polymers (i.e. polymer crosslinked with borate compounds) is their thermal stability (see SPE Paper 18 862). Many crosslinked polymers degrade at temperatures of 180.degree. F. Other crosslinkers, such as certain chromuim compounds are falling into disuse because of their known toxicity.
In order to avoid the undesirable after effects of the polymer and polymer residue, it is now common practice to employ in the fracturing fluid chemicals ("breaker") which degrade the polymers. U.S. Pat. No. 4,741,401 discloses a number of oxidizing agents contained in capsules for breaking the fracture fluid. U.S. Pat. No. 3,938,594 discloses the use of sodium hypochlorite solution, acid, micellar solutions, and surfactants for degrading the fracturing fluid polymers. As described in detail in SPE Paper 18862, published March 13-14, 1989, some breakers in fracturing fluids for shallow, low temperature (100.degree. F.) treatments are satisfactory for certain polymer gels.
These conventional breakers, however, are not particularly effective with organometallic crosslinked polymers. Moreover, in deep high temperature wells, particularly wells at temperatures in excess of 250.degree. F., many breakers tend to degrade the polymer prior to completion of fracture generation phase of the operation.
Finally, the aqueous fracturing fluid must not cause damage to the formation by swelling formation clays. Formations typically contain clays, particularly montmorillinite, that have the property of swelling when wet. Clay swelling can reduce formation fluid permeability.
In summary, the fracturing fluid must be formulated to: (a) achieve sufficient viscosity, preferably delayed viscosity to minimize friction loss; (b) possess high temperature stability; and (c) have non-damaging properties. As described in detail below, the present invention employs a crosslinked copolymer which exhibits these properties.
Prior art references which disclose copolymers used in well operations are: U.S. Pat. No. 4,137,182 discloses acrylamide/methacrylate copolymer crosslinked with chromium ions (crosslinker) in presence of carbonate ion activator, oxylate ion (stabilizer), and persulfate ion (breaker);. U.S. Pat. No. 3,938,594 discloses acrylamide/Na.sup.+ acrylate copolymers crosslinked with chromium ions; U.S. Pat. No. 4,210,206 discloses acrylamide/ammonium acrylate copolymers crosslinked with dialdehyes; and U.S. Pat. No. 4,541,935 discloses titanium and chromuim crosslinkers for a variety of polymeric thickeners.